Turbines typically comprise a rotating part such as the rotor, and a stationary part such as a stator. The rotor of a turbine may include a plurality of stacked wheels. The outer radial region of the stacked wheels is known as rim portion while the central radial region of the stacked wheels is known as the bore portion. Typical operation of turbines involves high temperatures which can subject the various components of the turbine to relatively extreme thermal loads. The rim portion of a rotor of a gas turbine is typically exposed to a high temperature gas flow, and therefore, is heated to relatively high temperatures compared to the bore portion of the rotor during start up, thus generating a significant radial thermal gradient between the rim and the bore of the rotor. This rim-to-bore thermal gradient causes stress cycling during startup and shutdown cycle of gas turbine and thereby adversely affects the life of the mechanical components of the turbine.
Moreover, typically in turbine rotor construction, a plurality of solid and annular wheels are placed in adjacent positions, and are secured to one another by a plurality of axially extending bolts and rabbeted joints provided between the adjacent wheels. Thus, differential heating of the wheels can cause significant rotor bore stresses and deflections which tend to open up the rabbeted joints. Moreover, the thermal conditions of the rotor and the associated wheels are different at startup, steady state operation, and turbine shutdown. During startup, the rim portion of the turbine wheels are typically in direct contact with the hot turbine flow path, and therefore, the rim portion tends to heat up faster than the bore portion, resulting in a relatively high temperature gradient. During steady state operation, heat from the rim portion is conducted to the bore portions, reducing the temperature gradient, and nearly equalizing the temperature differential between the rim and the bore. However, since the rim remains in direct contact with the hot gases, the temperature of the rim tends to be slightly elevated compared to the temperature of the bore, even at steady state. During shutdown, the temperature gradient tends to reverse because the reduced flow path temperature of the compressor section of turbine cools the rim portion, whereas the bore portion still retains the heat because of thermal inertia.
Thus there is a need for a design in which flow path or heated/cooled air can be directed to the turbine bore during respective startup and shutdown, and it can either be shutoff or drastically reduced during steady state operation. Accordingly, there is a need for methods, systems and apparatuses for passive purge flow control in a turbine.